1. Field of the Invention
The invention relates to a circuit arrangement for a picture display device having a cathode-ray tube whose final anode voltage is produced by a voltage source including a winding of a transformer and a rectifier connected to one end of the winding, the other end of the winding being coupled to a reference potential.
2. Description of Related Art
Picture display devices having cathode-ray tubes are generally known, for example, as television receivers or also as peripheral devices for computers for the processing of, for example, text and graphics. In these applications the high final anode voltage is recovered from the retrace pulses of the line transformer to which also the horizontal deflection coils are connected. A voltage source of this structure producing the final anode voltage has, however, a relatively high internal resistance. A variation of this voltage results, as is known, in a change of the size of the picture displayed on the display screen.
Picture display devices for connection to computers for, more specifically, word processing often operate on the so-called "positive display" principle, i.e. the characters, such as letters, digits etc. are displayed in a dark color on a bright background, as is normally also the case on paper. Then substantially the major portion of the picture surface is brightly illuminated, so that an anode current of highly different values flows in the cathode-ray tube in dependence on the adjusted picture brightness.
Frequently used measures for stabilizing the picture size utilize a voltage source having a relatively high internal resistance to feed the deflection coils, from which also the final anode voltage is derived, so that at a higher anode current, also the amplitude of the deflection signal becomes less, but when the anode voltage changes also the geometry of the electron beam changes, more specifically the dot sharpness.